1. Field of the Invention
The present invention relates to an object, a method, or a manufacturing method. The present invention relates to a process, a machine, manufacture, or a composition of matter. In particular, the present invention relates to, for example, a semiconductor, a semiconductor device, a display device, a light-emitting device, a lighting device, a power storage device, a memory device, or a processor. Furthermore, the present invention relates to a manufacturing method of a semiconductor, a semiconductor device, a display device, a light-emitting device, a lighting device, a power storage device, a memory device, or a processor. The present invention relates to a driving method of a semiconductor device, a display device, a light-emitting device, a lighting device, a power storage device, a memory device, or a processor.
In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A display device, a light-emitting device, a lighting device, an electro-optical device, a semiconductor circuit, and an electronic device include a semiconductor device in some cases.
2. Description of the Related Art
Attention has been focused on a technique for forming a transistor using a semiconductor over a substrate having an insulating surface. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit and a display device. Silicon is known as a semiconductor applicable to a transistor.
As silicon used as a semiconductor of a transistor, any of amorphous silicon, polycrystalline silicon, single crystal silicon, and the like is used depending on the purpose. For example, in the case of a transistor included in a large display device, it is preferable to use amorphous silicon, which can be used to form a film on a large substrate with the established technique. On the other hand, in the case of a transistor included in a high-performance display device where a driver circuit and a pixel circuit are formed over the same substrate, it is preferable to use polycrystalline silicon, which can be used to form a transistor having a high field-effect mobility. Furthermore, in the case of a transistor included in an integrated circuit or the like, it is preferable to use single crystal silicon which provides a much higher field-effect mobility. As a method for forming a film using polycrystalline silicon, high-temperature heat treatment or laser light treatment that is performed on amorphous silicon has been known.
In recent years, an oxide semiconductor has attracted attention. An oxide semiconductor can be formed by a sputtering method or the like, and thus can be used for a semiconductor of a transistor included in a large display device. Furthermore, a transistor including an oxide semiconductor has a high field-effect mobility; therefore, a high-performance display device where a driver circuit and a pixel circuit are formed over the same substrate can be obtained. In addition, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including amorphous silicon can be retrofitted and utilized.
A transistor including an oxide semiconductor is known to have extremely small leakage current in an off state. For example, a low power consumption CPU and the like utilizing the feature of small leakage current of the transistor including an oxide semiconductor are disclosed (see Patent Document 1). In the case where transistors each including an oxide semiconductor are used in an integrated circuit such as a CPU, the transistors are preferably decreased in size to be integrated.
When the degree of integration is increased in a semiconductor device, parasitic capacitance formed by overlap between wirings, electrodes, and the like might have a non-negligible effect. According to Patent Document 2, even with an offset region, a disclosed transistor achieves excellent electrical characteristics by electron injection from a conductor electrode to a semiconductor. By the technique disclosed in Patent Document 2, the parasitic capacitance formed by overlap between wirings, electrodes, and the like can be reduced.
It is also disclosed that a transistor having a high field-effect mobility can be obtained by a well potential formed using an active layer including a semiconductor (see Patent Document 3).